1. Field of the Invention
This invention relates to poly-N-cyanoimides which are useful as monomers for synthesizing polyimides and a process for preparing the N-cyanoimides. This invention also relates to a process for preparing polyimides, a process for curing epoxy resins, epoxy resin-cyanoimide compositions, reactive diluent-cyanoimide-amine compositions, poly(amide-cyanoamide) compositions, reactive diluentpoly(amide-cyanoamide) compositions, epoxy resin-polyimide compositions, bis-cycloalkyl dianhydrides, a process for preparing bis-cycloalkyl dianhydrides, and bis-cycloalkyl dianhydride based polyimides.
2. Description of the Related Art
Polymers containing imide groups are widely recognized and utilized for their heat resistant properties. Imide groups have been incorporated into linear thermoplastics, into oligomers end-capped with reactive groups such as acetylene or nadimide, and into crosslinkable thermoset monomers such as bismaleimides. Commercial imide polymers are available, for example, in the form of high T.sub.g thermoplastics, as solutions of the precursor poly(amide-acid), as reactive oligomers, as solutions of the amine and ester-acid precursors which are processed in-situ to a crosslinked polyimide oligomer, and as low molecular weight bismaleimide thermoset monomers. They have been utilized as resins for fiber-reinforced composites, as engineering thermoplastics, as coatings, as adhesives, and in other applications requiring high temperature stability.
Polyimides are generally formed from the appropriate anhydride and amine precursors via a two-step reaction sequence which involves ring opening addition of the amine to the anhydride to form an amide-acid followed by either thermal or chemical dehydration and ring closure to form the imide and remove an equivalent of water. The first step in this sequence is a facile reaction which occurs readily even at room temperature while the second step requires more rigorous conditions to force the elimination of water. In some cases the anhydride may also be replaced with the corresponding ester-acid.
If the final cyclization/dehydration step is performed prior to fabrication of the desired composite, adhesive bond or other application, the resulting imide polymer is often characterized by poor solubility and/or high melt-processing temperatures. If this dehydration step is carried out as a curing process during product fabrication, the water produced by cyclization must be carefully removed to eliminate voids or other defects in the final product. Such processing and curing requires high temperatures and/or vacuum plus extended time periods, all of which add to the cost of reliably producing polyimide-based products. Furthermore, while reactive end-capped polyimide oligomers or higher molecular weight poly(amide-acids) tend to be more soluble than the corresponding high molecular weight polyimides and are therefore more readily processed as solutions, this carrier solvent must also be removed at some point in the fabrication process. Thus, most of the polyimide systems now in use require the removal during processing of either solvent or water, or both.
All these problems arise from the high T.sub.g values and low solubilities of most imide containing polymers, the very features which, in turn, make polyimides attractive. The commercial systems now in use have all evolved as particular solutions to this problem of combining outstanding final physical properties with acceptable processing requirements.
N-cyanoimides in general and poly-N-cyanoimides in particular have been found to function as "anhydride equivalents" in imide forming reactions and to provide significant advantages over the anhydrides and other precursors currently used. In particular, the temperatures required for final ring closure to the imide are lower and the by-product in this case is a non-volatile solid. The most advantageous application of the use of poly-N-cyanoimides in the preparation of polyimides is thus in those cases where the polyimide is formed during final part fabrication and where the use of N-cyanoimides can eliminate the need to remove water from the final product.
Polyfunctional N-cyanoimides are unknown in the prior art. However, certain monofunctional N-cyanoimides have been prepared. For example, diacetylcyanamide (N-cyanodiacetamide) [J. Prakt. Chem., 17(2), 14, (1875)]; bis-(4-hydroxy-3,6-dioxohexahydropyridazinyl-(4)-acetylcyanamide (German patent No. 2,356,368); N-cyanosuccinimide [J. Prakt. Chem., 22(2), 193 (1880)]; N-cyanophthalimide, 4-nitro-N-cyanophthalimide [Zh. Org. Khim., 13(5), 968 (1977), Chemical Abstracts 87: 68071n] have been prepared. None of the methods used to make the preceding compounds uses the process of the present invention or discloses the polyfunctional N-cyanoimides.
The polymeric intermediates obtained by reaction of a bis-N-cyanoimide and a diamine to form a poly(amide-cyanoamide) are unknown in the prior art. These polymeric compositions are analogous to the known poly(amide-acid) intermediates, but may be converted to the polyimide with the elimination of cyanamide instead of water. The process for converting poly(amide-cyanoamides) to polyimides is also unknown in the prior art.
The use of mono- or polyfunctional N-cyanoamides to cure epoxy resins is unknown in the prior art. Curable compositions comprising an epoxy resin and a mono- or polyfunctional N-cyanoimide are unknown despite the long history of dicyandiamide and other cyanamide derivatives as epoxy curing agents.
The use of other derivatives of cyanamide, or its dimer, as curing or hardening agents for epoxy resins is known in the art. U.S. Pat. No. 4,168,364 teaches cyanamides of organic primary amines as epoxy curing agents. U.S. Pat. Nos. 4,379,728 and 4,384,084 teach the uses of N-cyanourea compounds and cyanolactams respectively. Curable compositions containing N-cyanoamides and polyepoxy resins have been described in both U.S. Pat. Nos. 4,435,549 and 4,618,712. U.S. Pat. No. 4,859,761 discloses epoxy resin compositions containing specific cyanoguanidines as latent hardeners.
Also unknown are curable compositions comprising a reactive diluent such as an epoxy resin plus a bis-N-cyanoimide and a diamine and curable compositions comprising a reactive diluent and a poly(amide-cyanoamide).
N-Cyanoimides derived from bis-cycloalkyl dianhydrides are especially effective in certain embodiments of this invention. The bis-cyclohexyl dianhydrides of this invention are also unknown in the prior art as are the polyimides derived from them. Processes for the preparation of the bis-cycloalkyl dianhydrides have also been described only partially in the prior art.
European Patent Application 0 311 374 teaches that dicyclohexyl-3,4,3',4'-tetracarboxylic acid can be made by hydrogenating biphenyl-3,4,3',4'-tetracarboxylic acid tetramethyl ester in a solvent such as methanol, methyl acetate, tetrahydrofuran, diethyl ether, or n-hexane in the presence of a rhodium catalyst at a temperature of from 50.degree. C. to 150.degree. C. at a hydrogen pressure of from 2 to 100 atmospheres followed by hydrolysis of the reduced esters to the tetracarboxylic acid under well known acid or alkaline conditions. The patent application also teaches that the corresponding dicyclohexyl-3,4,3,4'-tetracarboxylic dianhydride can be made by dehydrating the dicyclohexyl-3,4,3',4'-tetracarboxylic acid by either heating the acid to 180.degree. C. to 220.degree. C. under a pressure of 30 to 100 mm Hg for 1 to 5 hours or refluxing the acid with acetic anhydride.
European Patent Application 0 311 374 also teaches polyimides having the following repeat unit ##STR1## wherein R is a bivalent group can be made by reacting dicyclohexyl-3,4,3,4'-tetracarboxylic dianhydride with a diamine to produce a poly(amide-acid) and dehydrating the poly(amide-acid). U.S. Pat. No. 3,600,406 teaches that bis(4-cyclohexyl-1,2-dicarboxylic anhydride) ketone can be made by hydrocarboxylation of 4-cyclohexene-1,2-diethyl carboxylate with nickel carbonyl or carbon monoxide followed by hydrolysis to the tetracarboxylic acid ketone and subsequent dehydration with acetic anhydride.
Japanese patent number 01,238,576 teaches that bis(1,2-cyclohexyldicarboxylic dianhydride) tetradecene can be made by reacting tetrahydrophthalic anhydride and 1,13-tetradecadiene at 200.degree. C. in a nitrogen atmosphere.